1. Field of the Invention
An electro-mechanical energy conversion system including a doubly fed induction machine to selectively convert and transfer energy from an energy source and an energy load,
2. Description of the Prior Art
Seemingly limitless electro-mechanical systems and devices have been devised to convert electrical energy to mechanical energy or vice versa.
In efforts to reduce dependency on fossil fuels, countless generator systems to convert mechanical to electrical energy including various wind power systems or wind turbines have been developed. Such systems generally include a shaft-mounted turbine to drive an electrical generator. To operate the generator at optimum speed for maximum power output, the wind turbine must produce a relatively constant torque or speed despite changes in wind speed and wind direction. Generally, the pitch of the turbine blades is varied to regulate the torque or resultant speed. Unfortunately, such pitch angle control mechanisms are complex and costly to manufacture, maintain and repair.
Moreover, such energy conversion systems using variable speed wind turbine generators to provide the source of energy to utility power grids require a matched constant output frequency at preferably optimum power output. Thus, the variable frequency AC from such turbine generators must be converted to a constant frequency AC for use by the utility power grid. Generally, this conversion can be accomplished through an intermediate conversion to DC by a rectifier and subsequent inversion to fixed-frequency AC by means of an inverter. Unfortunately, such systems are inefficient, relatively expensive, difficult to maintain operation and relatively unreliable as an electricity source.
U.S. Pat. No. 5,028,804 discloses an energy conversion generation system to receive energy from a resource and convert the energy into electrical power for supply to a polyphase electrical power grid operating at a system frequency. The controller establishes a reference signal, then processes the sensor signal with the reference signal to produce a controller signal. The converter produces the excitation power at an excitation frequency in response to the controller signal so as to increase the ratio of the electrical power output to the resource energy power input received by the prime mover.
U.S. Pat. No. 4,490,093 describes a windpower system comprising a support and a turbine having a shaft rotatively mounted to the support. The turbine has variable pitch blades controlled by the differential motion of a rotary control shaft coaxial with the turbine shaft and the turbine shaft so that the blade pitch can be varied by a stationary motor without requiring any slip rings or other such wear prone couplings. In the event of a power failure, rotary motion of the control shaft is prevented so that the turbine blades are feathered solely due to the force developed by the rotating turbine. When the turbine is used to generate electrical power an induction generator is coupled to the turbine shaft. The shaft speed is indicative of generator output power. Thus, generator speed is monitored and used to control the pitch of the turbine blades so as to maintain generator output power at the maximum value when wind speed is below the machine's rated wind speed and no more than rated output power when wind speed exceeds rated wind speed.
U.S. Pat. No. 4,426,192 teaches a method and apparatus for controlling windmill blade pitch. The pitch of the turbine blades is based on a dual-deadband control strategy. If the current turbine speed is determined to be outside of a relatively wide deadband, action is taken to correct the speed by changing blade pitch. If the current speed is within the relatively wide deadband, then the average of the turbine speed over an interval is compared with a relatively narrow deadband within the wider deadband. Action is then taken to change the blade pitch if the average speed is outside the narrow deadband. In this way, wide excursions of turbine speed are corrected promptly, but the frequency of control actions is minimized by requiring only the average speed to be kept within tight limits.
U.S. Pat. No. 6,137,187 relates to a variable speed system such as a wind turbine comprising a wound rotor induction generator, a torque controller and a proportional, integral derivative (PID) pitch controller. The torque controller controls generator torque using field oriented control, and the PID controller performs pitch regulation based on generator rotor speed
U.S. Pat. No. 5,225,712 shows a wind turbine power converter that smooths the output power from a variable speed wind turbine to reduce or eliminate substantial power fluctuations on the output line. The power converter has an AC-to-DC converter connected to a variable speed generator that converts wind energy to electric energy, a DC-to-AC inverter connected to a utility grid, and DC voltage link connected to an electrical energy storage device such as a battery or a fuel cell. An apparatus and method for controlling the instantaneous current flowing through the active switches at the line side inverter to supply reactive power to the utility grid is also disclosed. The inverter can control reactive power output as a power factor angle, or directly as a number of VARs independent of the real power. Reactive power can be controlled in an operating mode when the wind turbine is generating power, or in a static VAR mode when the wind turbine is not operating to produce real power. To control the reactive power, a voltage waveform is used as a reference to form a current control waveform for each output phase. The current control waveform for each phase is applied to a current regulator which regulates the drive circuit that controls the currents for each phase of the inverter. Means for controlling the charge/discharge ratio and the regulating the voltage on the DC voltage link is also disclosed.
U.S. Pat. No. 5,028,04 relates to an energy conversion generation system to receive energy from a resource and convert the energy into electrical power for supply to a polyphase electric power grid operating at a system frequency. A prime mover driven by the resource energy and a converter such as a power electronic converter produces excitation power from power received from a converter power source. A brushless doubly-fed generator having a rotor with rotor windings and a stator with stator windings comprises a first and second polyphase stator system. The rotor is driven by the prime mover. The first stator system supplies the electrical power to the grid, and the second stator system receives the excitation power from the converter. A sensor senses a parameter of the electrical power output supplied to the grid and produces a sensor signal corresponding to the sensed parameter. A controller controls the converter in response to the sensor signal.
U.S. Pat. No. 4,523,269 discloses a DC to N phase AC converter, a DC source having first and second terminals for deriving equal amplitude opposite polarity DC voltages, a series resonant circuit, and N output terminals, one for each phase of the converter. The series resonant circuit is selectively connected in series with the first and second terminals and the N output terminals for an interval equal to one half cycle of the resonant circuit resonant frequency, so that current flows between a selected one of the first and second terminals and the resonant circuit and a selected one of the N output terminals during the interval. The resonant circuit current is zero at the beginning and end of the interval. A capacitor shunting each of the output terminals has a value relative to the capacitance of the series resonant circuit such that the voltage across each output terminal remains approximately constant between adjacent exchanges of energy between the resonant circuit and the output terminal. The selective connection is in response to a comparison of the actual voltage across each of the N output terminals and a reference voltage for each of the N output terminals. The comparison controls when the flow of current between the selected first and second terminals and the selected output terminal via the resonant circuit begins. The frequency of the AC voltage developed across the N output terminals is much less than the resonant frequency of the circuit.
Despite these systems, there remains a need for an efficient, reliable variable speed energy conversion system.